Cable television networks such as those provided by Comcast Cable Communications, Inc., of Philadelphia, Pa., Cox Communications of Atlanta Ga., Tele-Communications, Inc., of Englewood Colo., Time-Warner Cable, of Marietta Ga., Continental Cablevision, Inc., of Boston Mass., and others provide cable television service to a large number of subscribers over a large geographical area. The cable television networks typically are interconnected by cables such as coaxial cables or a Hybrid Fiber/Coaxial (“HFC”) cable system. The system can also provide data services having data rates of about 10 Mega-bits-per-second (“Mbps”) to 30+ Mbps per channel.
The Internet, a world-wide-network of interconnected computers, provides multi-media content including audio, video, graphics and text that requires a large bandwidth for downloading and viewing. Most Internet Service Providers (“ISPs”) allow customers to connect to the Internet via a serial telephone line from a public switched telephone network at data rates including 14,400 bps, 28,800 bps, 33,600 bps, 56,000 bps and others that are much slower than the about 10 Mbps to 30+ Mbps available on a coaxial cable or HFC cable system on a cable television network.
With the explosive growth of the Internet, many customers have desired to use the larger bandwidth of a cable television network to connect to the Internet and other computer networks. Cable modems, such as those provided by 3Com Corporation of Santa Clara, Calif., and others offer customers higher-speed connectivity to the Internet, an intranet, local area networks (“LANs”) and other computer networks via cable television networks. These cable modems currently support a data connection to the Internet and other computer networks via a cable television network with a data rate of up to 30+ Mbps which is a much larger data rate than can be supported by a modem used over a serial telephone line.
Background information related to cable modem systems in general is described in the Data-Over-Cable Service Interface Specifications (“DOCSIS”)—Radio Frequency Interface Specifications, Interim Draft, dated Jul. 24, 1998, issued by Cable Television Laboratories, Inc. DOCSIS may be found on the World Wide Web at the Universal Resource Locator (“URL”) “www.cablemodem.com”. This document, known to persons working in the art, is incorporated by reference herein in its entirety.
The basic overall architecture of a data-over-cable system is shown in FIG. 1. The system of FIG. 1 provides a mechanism by which a computer 10 connected to a backbone network 12 (either directly or indirectly by intermediate networks) may communicate with another computer 14 via a cable television infrastructure indicated generally by reference numeral 16. The cable television infrastructure 16 includes a distribution hub or “head-end” 18 that is connected to the backbone network 12 via a wide area network (“WAN”) and a switch or router 20. A cable system head-end 18 is a central location in the cable television network that is responsible for sending cable signals in the downstream direction. The head-end 18 modulates digital data into analog form and supplies analog signals to a fiber network 22, which is connected to a plurality of optical/electronic (“O/E”) nodes 24. The O/E nodes 24 convert optical signals in the fiber network 22 to electrical signals for transmission over a coax cable network 26 to a cable modem 28 at the customer's location. The cable modem 28 demodulates the analog signals and extracts the digital data and supplies the data to the customer premise equipment (“CPE”) 14, which, in a typical situation, is a general purpose computer in a home environment.
The head-end 18 includes a cable modem termination system (“CMTS”) 30. This device provides a network side interface to a wide area network, indicated at 32, and a radio frequency (“RF”) interface between the cable modem termination system and the cable network in both the downstream and upstream directions, indicated at 34 and 36. The term “downstream”, as used in the present document, refers to transmission in the direction from the head-end 18 or cable modem termination system 30 to the cable modem 28 at the customer premises. The term “upstream” refers to transmission in the direction from the cable modem 28 at the customer premises to the cable modem termination system 30.
For transmission in the downstream direction, the cable modem termination system 30 supplies data from the computer 10 to a modulation circuit (“MOD”) and to a combiner 38, where the data is combined with video signals for the cable television system. The combined signals are sent to a transmission module 40 where they are imparted onto the fiber network. In the receiving direction, data from the CPE 14 is received from the fiber network at a receive module 42, sent to a splitter and filter bank 44 and sent to a demodulation circuit (“DEMOD”) in the cable modem termination system 30. The data is processed by a network termination unit 46, sent to the switch or router 20 and routed onto the WAN for transmission to the remote computer 10.
Many cable television networks provide only unidirectional cable systems, supporting only a “downstream” cable data path. A return data path via a telephone network (i.e., a “telephony return”), such as a public switched telephone network provided by AT&T, GTE, Sprint, MCI and others, is typically used for an “upstream” data path. A cable television system with an upstream connection to a telephony network is called a “data-over-cable system with telephony return.” Such a return system is indicated at 48 where the cable modem 28 is also shown connected to the public switched telephone network (“PSTN”).
An exemplary data-over-cable system with telephony return includes customer premises equipment (e.g., a customer computer), a cable modem, a cable modem termination system, a cable television network, a public switched telephone network, a telephony remote access concentrator (“TRAC”) 49 and a backbone data network 12 (e.g., the Internet). The cable modem termination system 30 and the telephony remote access concentrator 49 together are called a “telephony return termination system.”
In a two-way cable system without telephony return, also termed a bi-directional cable system, the customer premises equipment 14 sends data packets to the cable modem 28, which sends the data packets upstream via the cable television network 22 and 26 to the cable modem termination system 30. Such a system is shown in FIG. 1. The cable modem termination system 30 sends the data packets to appropriate hosts on the data network 12. The cable modem termination system 30 sends the response data packets back to the appropriate cable modem 28.
In a bi-directional cable system, the cable modem termination system 30 can continuously collect information about the level of impairments on the upstream RF path of a cable plant, i.e., the portion of the network between the demodulation circuit in the cable modem termination system 30 and the cable modems 28. Further, a single O/E node 24 may serve multiple channels and cable modems. Measurements such as the noise floor level, and signal-to-noise ratio per cable modem transmission, can be made for the coax and fiber networks, along with the tracking of which cable modems are active during a given measurement interval.
The data-carrying performance of the upstream channels may vary with the conditions for radio frequency propagation on the cable network. Defective radio frequency interfaces may introduce sufficient noise into an upstream channel that the noise significantly impairs the ability of the channel to transport data packets from the cable modems to the cable modem termination system without error. Extraneous sources of radio frequency, such as citizen band or amateur radio broadcasts, may also infiltrate the upstream channels, interfere with the radio frequency carriers for the upstream channels, increase the packet error rate, reduce the data throughput, and generally impair the performance of the data-over-cable network.
Previous methods for improving performance have included switching the frequency of a degraded upstream channel to another part of the upstream radio frequency spectrum on the cable. The cable modems on the degraded upstream channel are all instructed to hop to a clean frequency. However, the methods are ineffective when the degradation is present over the full spectrum of upstream frequencies, as there are no cleaner frequencies than the current operating frequency.
Previous methods also typically retain the same data transmission configurations for the cable modems when there are other configurations of the cable modems that may optimize throughput for the current network conditions. In many instances, static configurations for the cable modems may actually limit throughput relative to what could be achieved. In previous methods, the configurations of the cable modems are determined at the time the cable modems initialize or the configurations are preset at the factory. The configuration parameters include the type of radio frequency modulation, the type and extent of error correction, and the symbol rate. In particular, static configurations cannot compensate for an intermittent degradation or a reconfiguration of the network and network paths. As the degradation impinges on the network, the current configurations of the cable modems may be different from those that give the optimal throughput of data. Dynamic configuration of the cable modems may allow the network to attain its optimal performance.
It is therefore desirable to improve the configuration of the cable modems so that upstream performance is optimized for the present network conditions. Optimizing data throughput may improve the performance of the data-over-cable network.